Gemini North Mirror Receives New Silver Coating During Major Shutdown

August 14, 2008

During the second half of July 2008, the Gemini North telescope facility
on Mauna Kea underwent a major 20-day scheduled shutdown. The shutdown
allowed engineers and technicians to address a number of deferred upgrades and maintenance projects, (see a list of projects here), that are only possible during an extended period of electromechanical
work. Included was the successful re-coating of the 8-meter primary
mirror with protected silver.

Figure 1. Claudio Araya, Christian Aguilar, and Hector Figueroa standing on the wash bridge during the initial phase of stripping the old coating off of the Gemini North telescope's primary mirror.

"As a scheduled shutdown, we are able to take care of maintenance and
upgrades that are not possible when we are on the sky each night," said
Brian Walls of Gemini's Systems Engineering team. "And while the new
silver coating gets most of the attention, the concurrent work done on
the telescope is critical since the entire telescope system needs to function at the same high level..."

Figure 2. First look at the new silver coating on the Gemini North telescope's primary mirror.

About four years ago the Gemini North telescope's primary mirror received its first
protected silver coating and during this period its durability has exceeded expectations and
compares favorably more traditional aluminum coatings used on other
large telescopes. The new coating was completed on July 29th and testing
has confirmed that it has slightly better reflectivity than the previous
coating (See Figure 3). After testing and calibrations, the mirror
resumed collecting light for science observations on the night of August
3rd.

Figure 3. Plot of current Gemini North telescope's primary mirror reflectivity since
last coating in 2004. Note that the most recent coating is indicated by the
points at upper right.

The new coating restores the high reflectivity of the Gemini North
mirror in the optical (visible) part of the spectrum as well as the low
thermal emissivity that silver provides. Low thermal emissivity is
critical to Gemini's exceptional performance in the mid-infrared part of
the spectrum. The emissivity of Gemini North telescope system (with prior mirror coatings, in December 2005) was ~5% at 12.10 microns (between sky lines). Updated values with the new coatings will be obtained once the mid-infrared instrument MICHELLE is scheduled back on the telescope.

Figure 4. Chris Yamasaki, Cooper Nakayama, and John White inspecting the PWFS1 module of the acquisition and guiding system before taking it to the instrument lab for servicing.

The shut-down team consisted of a maximum of 38 technicians, engineers
and support staff (13 from the Gemini South facility in Chile) who will
assemble again in 2009 to do a scheduled shutdown and recoating of the
Gemini South telescope's primary mirror. It is estimated that the entire shutdown
required some 11,000 person-hours of work in preparation and execution.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.